I. Field of the Invention
This invention relates generally to the field of sensors. In particular, the present invention applies to electronic systems that measure amplitudes of randomly arriving pulses, such as systems for in charge amplification and processing of signals from sensors.
II. Background of the Related Art
Detection is an established technique in the fields of nuclear medicine and astrophysics. In recent years, the use of detectors to image and characterize known as well as unknown sources has been advantageously applied to other fields such as biomedical research and the investigation of suspicious target materials at airports. By accurately sensing emitted signals from a target material, detection can be used to obtain information from concealed targets such as malignant tumors in human body, explosives, drug-based contraband, and nuclear-based contraband. Detection involves the processing of pulses, and inaccuracy may be introduced in the case where pile-up of these pulses, such as partial and/or complete superposition in time of these pulses, occurs.
In these systems pile-up, such as partial and/or complete superposition in time of considered pulses, may depend on signal rate and pulse width. Such pile-up generates distortion in the spectrum of a measured source due to pulse amplitudes arising from pile-up.
Pulse pile-up arises, for example, in conjunction with an increase in the event rate corresponding to events from a source. The time-of-arrival probability for corresponding pulses typically follows a Poisson distribution, with the interval between two events perhaps being expressed by the distribution function I1(t):I1(t)=r×exp(−r×t),where t is time and r is the average event rate. It is noted that the distribution indicates that the most probable interval between two events is zero. It is further noted that the distribution function indicates that as rate increases the probability of a short interval between events increases and pile-up occurs, with pile-up causing one or more peak detectors to measure one or more amplitudes higher than desired, such measurement generating spectral measurement distortion. The spectral measurement distortion limits the detection of spectral lines with amplitudes higher than the lines at a high rate.
Attempts to address pile-up of detected pulses have been made with limited success. Current approaches include those making use of digital signal processing. Such approaches are described, for example, in: W. Guo, S. H. Lee, and R. P. Gardner, “The Monte Carlo approach MCPUT for correcting pile-up distorted pulse-height spectra,” Nucl. Instrum. Meth., A531, pp. 520-529, 2004; V. T. Jordanov and G. F. Knoll, “Digital pulse-shape analyzer based on fast sampling of an integrated charge pulse,” IEEE Trans. Nucl. Sci., vol. 42, no. 4, pp. 683-687, 1995; M. W. Raad and L. Cheded, “Novel peak detection algorithms for pileup minimization in gamma ray spectroscopy,” IEEE Proc. Instrum. Meas. Tech. Conf., pp. 2240-2243, 2006; G. F. Grinyer, C. E. Svensson, C. Andreoiu, A. N. Andreyev, R. A. E. Austin, G. C. Ball, et al., “Pile-up corrections for high-precision superallowed β decay half-life measurements via γ-ray photopeak counting,” Nucl. Instrum. Meth., A579, pp. 1005-1033, 2007; F. Belli, B. Esposito, D. Marocco, M. Riva, and A. Zimbal, “Application of a digital pileup resolving method to high count rate neutron measurements,” Rev. Sci. Instrum., vol. 79, 2008; M. Bolic, V. Drndarevic, and W. Gueaieb, “Pileup correction algorithms for very-high-count-rate gamma-ray spectrometry with NaI(Tl) detectors,” IEEE Trans. Instrum. Meas., vol. 59, no. 1, pp. 122-130, 2010; U.S. Patent Application Publication No. 2009/0032715 of Mott; and U.S. Patent Application Publication No. 2009/0074281 of McFarland, et al., each of which is incorporated by reference in its entirety as if fully set forth in this specification.
Current approaches also include those making use of analog techniques. Such approaches are as described, for example, in: S. L. Blatt, J. Mahieux, and D. Kohler, “Elimination of pulse pile-up distortion in nuclear radiation spectra,” Nucl. Instrum. Meth., 60, pp. 221-230, 1968; S. Popov, A. Vastly, V. Garbusin, and E. Morozof, “CAMAC standard high speed precise spectrometer,” IEEE Nucl. Sci. Symp. Conf. Rec., pp. 375-378, 1995; T. Frizzi, L. Bombelli, C. Fiorini, and A. Longoni, “The SIDDHARTA chip: a CMOS multi-channel circuit for silicon drift detectors readout in exotic atoms research,” 2006 IEEE Nucl. Sci. Symp. Conf. Rec., pp. 850-856, 2007; B. Sabbah and I. Klein, “Pile-up rejection by comparison of the shaped pulse with its second derivative,” Nucl. Instrum. Meth., 95, pp. 221-230, 1968; G. Germano and E. J. Hoffman, “An investigation of methods of pileup rejection for 2-D array detectors employed in high resolution PET,” IEEE Trans. Medical Imaging, vol. 10, no. 2, pp. 223-227, 1991; A. Dragone, G. De Geronimo, J. Fried, A. Kandasamy, P. O'Connor, and E. Vernon, “The PDD ASIC: highly efficient energy and timing extraction for high-rate applications,” IEEE Nucl. Sci. Symp. Conf. Rec., pp. 914-918, 2005; and U.S. Pat. No. 6,573,762 to Wessensorf, et al., each of which is incorporated by reference in its entirety as if fully set forth in this specification.
Techniques making use of digital signal processing require fast Analog-To-Digital conversion (ADC) of the considered analog signal. Such techniques are described, for example, in W. Guo, S. H. Lee, and R. P. Gardner, “The Monte Carlo approach MCPUT for correcting pile-up distorted pulse-height spectra,” Nucl. Instrum. Meth., A531, pp. 520-529, 2004; V. T. Jordanov and G. F. Knoll, “Digital pulse-shape analyzer based on fast sampling of an integrated charge pulse,” IEEE Trans. Nucl. Sci., vol. 42, no. 4, pp. 683-687, 1995; and M. W. Raad and L. Cheded, “Novel peak detection algorithms for pileup minimization in gamma ray spectroscopy,” IEEE Proc. Instrum. Meas. Tech. Conf., pp. 2240-2243, 2006, each of which is incorporated by reference in its entirety as if fully set forth in this specification. The power needed to perform ADC in each channel and the time needed for digital processing are prohibitive for at least those applications that require high density of channels or that have a limited power budget.
The most commonly adopted analog techniques to address pulse pile-up use an additional fast shaper and veto logic to detect and reject pile-up in the main slow shaper. Such techniques are described, for example, in S. L. Blatt, J. Mahieux, and D. Kohler, “Elimination of pulse pile-up distortion in nuclear radiation spectra,” Nucl. Instrum. Meth., 60, pp. 221-230, 1968; S. Popov, A. Vastly, V. Garbusin, and E. Morozof, “CAMAC standard high speed precise spectrometer,” IEEE Nucl. Sci. Symp. Conf. Rec., pp. 375-378, 1995; and T. Frizzi, L. Bombelli, C. Fiorini, and A. Longoni, “The SIDDHARTA chip: a CMOS multi-channel circuit for silicon drift detectors readout in exotic atoms research,” 2006 IEEE NucL Sci. Symp. Conf. Rec., pp. 850-856, 2007, each of which is incorporated by reference in its entirety as if fully set forth in this specification. Such techniques require additional shaping and discrimination circuits.
Other analog techniques found in the literature are generally characterized with respect to the instant invention either by higher complexity or by the need for additional ADC and off-line processing. Such techniques are described, for example, in B. Sabbah and I. Klein, “Pile-up rejection by comparison of the shaped pulse with its second derivative,” Nucl. Instrum. Meth., 95, pp. 221-230, 1968; G. Germano and E. J. Hoffman, “An investigation of methods of pileup rejection for 2-D array detectors employed in high resolution PET,” IEEE Trans. Medical Imaging, vol. 10, no. 2, pp. 223-227, 1991; and A. Dragone, G. De Geronimo, J. Fried, A. Kandasamy, P. O'Connor, and E. Vernon, “The PDD ASIC: highly efficient energy and timing extraction for high-rate applications,” IEEE NucL Sci. Symp. Conf. Rec., pp. 914-918, 2005, each of which is incorporated by reference in its entirety as if fully set forth in this specification.
In view of these foregoing and other considerations, there is a need to develop improved approaches for addressing distortion in detected signals due to pulse pile-up.